1. Field of the Invention
This invention relates to an amorphous photovoltaic energy conversion element for use in an amorphous film solar cell.
2. Description of the Prior Art
The amorphous film solar cell is expected to lower fabrication costs compared with that for single-crystalline photovoltaic energy conversion elements and to enjoy a steady growth in demand. Therefore, the amorphous photovoltaic energy conversion element is attracting mounting attention for use in an amorphous film solar cell.
At first, the photovoltaic energy conversion element was developed as a p-n junction element of two-layer construction. However, the amorphous film p-n junction element has the disadvantage that it fails to provide photovoltaic energy conversion with high efficiency because only carriers generated in the depletion layer by the incident light are converted into electric power.
As an improvement in the aforementioned two-layer element, there has been proposed a three-layer element having an intrinsic layer (i-layer) of amorphous semiconductor interposed between a p-layer and an n-layer, and in the intrinisic layer, carriers generated by the light are converted into electric power (D. E. Carlson et al, Applied Physics Letters, Vol. 28, No. 11, June 1, 1976).
In the three-layer element of the foregoing description, when the second layer constituting an intermediate layer forms a built-in potential of required gradient throughout the entire length thereof, as illustrated in FIG. 1, this i-layer will be wholly utilizable, thereby implying that the efficiency of photovoltaic energy conversion will theoretically be appreciably improved. In the figure, E.sub.f denotes Fermi level.
Also, in such a three-layer construction as described above, the materials for layer I and layer III and physical and electrical properties thereof are chosen consistent with the theory that this three-layer element is an advanced version of the p-n junction element. As a natural consequence, a p-type material has been selected for layer I and an n-type material has been selected for layer III. This selection has never been brought into question.
Besides, in the actual manufacture of an amorphous photovoltaic conversion element of this nature, layer III (the n-layer) is generally formed last during the process of depositing the three layers. It may be safely concluded that this sequence of formation of the three layers will not be changed at least for the time being.
A conventional amorphous film solar cell fabricated under such restrictions regarding the selection of materials and the mode of production is illustrated in FIG. 2. Referring to this diagram, a solar cell 8 is completed by first forming a transparent electrode 6 on a glass substrate 5 and thereafter superposing an amorphous photovoltaic energy conversion element 1 on the transparent electrode. To be more specific, it has been customary to form an amorphous film solar cell 8 as a whole by superposing a p-layer 2 as layer I and an i-layer 3 as layer II in the order mentioned on the transparent electrode 6 and superposing an n-layer 4 as layer III on the i-layer 3, thereby producing the amorphous photovoltaic energy conversion element 1, and thereafter forming a suitable metal film 7 on the n-layer 4 in ohmic contact therewith. The incident light l passes through the glass substrate 5 and the transparent electrode 6 and enters the i-layer from the p-layer side. Unfortunately, the conventional amorphous photovoltaic energy conversion element 1 constructed as described above has the drawbacks mentioned below.
(1) Since amorphous semiconductors are readily affected by impurities, the separate formation of the p-, i-, and n-layers requires the use of a separate film-forming chamber for each of the three layers. Further, in the conventional construction of the three-layer element, the provision of the electrode on the n-layer side requires the formation of a metal film. Thus, the procedure and the equipment for the production become complicated.
(2) For layer III which borders on the i-layer, an n-type semiconductor has been selected, as already mentioned. Since the n-layer is devoid of any built-in potential gradient, the electron-hole pairs generated in this layer are not able to separate but recombine despite the incidence of light. Thus, these electron-hole pairs cannot be utilized for output current.
(3) Even with the latest technology, the formation of the n-layer can be carried out only by use of such a highly poisonous gas as PH.sub.3 and the process of producing the element consequently includes a dangerous step.